Spain's Constitutional Court has suspended Catalonia's planned independence referendum.
The court said it first needed to consider arguments whether the 9 November vote breached the country's constitution.
It acted on a request from the Spanish central government in Madrid.
The leader of Catalonia, Artur Mas, signed a decree on Saturday calling for the referendum. Many in the wealthy north-eastern Spanish region back him.
But Spanish Prime Minister Mariano Rajoy told reporters that the vote was not "compatible with the Spanish constitution".
"Nobody and nothing will be allowed to break up Spain."
He was speaking in a televised statement to the nation after holding an emergency cabinet meeting.
Hundreds of thousands of Catalans joined a protest in Barcelona recently, calling for their right to vote.
Unhappy at Spain's refusal to give Catalans more powers, protesters have been energised by Scotland's recent independence referendum, and many also waved the Scottish flag.
Catalonia's 7.5 million inhabitants make up approximately 16 % of the population of Spain. Yet it is one of Spain's richest and most highly industrialised regions, as well as one of its most independent-minded.
Spain's deepening economic crisis, though, has seen a surge in support for separation.
On 11 September a vast crowd rallied in support of independence in Barcelona, as Catalonia marked "La Diada", the 300th anniversary of the end of the city's siege.
Then on 19 September the regional parliament voted by 106 to 28 in favour of granting Catalonia's president the power to hold a referendum, known locally as a "consultation".
Mr Mas, who was re-elected in December 2012, says he can use local laws to hold the vote, even though the central government says its blessing is required.
"I can't pretend that this will be easy, but it doesn't work just coming out to protest once a year," Mr Mas said on Catalan television at the weekend.
"The future is something you conquer, not a gift, and we have to earn that," he added.
A recent poll for Spain's El Pais newspaper showed that 45% of Catalans were in favour of suspending the referendum if the Constitutional Court declared it illegal.
Only 23% would like the referendum to go ahead regardless, the survey suggested.
Mr Mas has only recently become a supporter of full independence. Since 2007, he has spearheaded a push to revitalise Catalan nationalism known as the Refoundation of Catalanism.